The present invention relates generally to satellite control systems and more particularly to nutation control systems for dual-spin stablized satellites.
In many satellite applications, it is desirable and necessary to precisely maintain the satellite in a predetermined orientation or attitude with respect to a given reference direction. This may be particularly true for communications satellites in geosynhcronous orbit where an antenna must be maintained in a given orientation toward a ground station. One well-known method of maintaining the orientation of a satellite is by spinning the satellite about an axis of symmetry. To provide a portion of the satellite, such as an antenna, with a relatively constant orientation with respect to an outside reference direction, one portion of the satellite may be spun while another portion on which the antenna is mounted is despun. Thus, a dual-spin stabilized satellite is characterized by a rotor or spun portion and a platform or despun portion, the two portions being coupled by despin motor and bearing assembly.
One problem with spin-stabilized satellites is that they may exhibit certain types of troublesome motions called "wobble", "precession" or "nutation". All such motions tend to result in a displacement of the satellite's geometric axis from its intended mission orientation or attitude.
Nutation of a satellite, or the coning motion of the spin axis about the total angular momentum vector, may result from any of the following disturbances: (1) booster final stage angular motion, (2) operation of separation equipment, (3) operation of payload components with uncompensated momentum, (4) motion of flexible elements of structure, (5) motion of liquids such as propellants or coolants, and (6) operation of mass expulsion devices on the spin-stabilized satellite.
In general, nutation is rotational motion about either or both of the transverse (non-spinning) axes which causes a rotational coning motion of the pitch (spin) axis about the total or resultant momemtum vector of the spacecraft. The rate of the coning motion is termed the nutation frequency and the cone angle of such motion is the amplitude of the nutation.
In order to maintain precise orientation of the satellite despun platform, nutation must be reduced or eliminated. The nutation of a conventional dual-spin satellite may be reduced by energy absorbing or momentum transfer devices operable on either or both of the transverse axes of the satellite to attenuate the nutation. Nutation effects may also be overcome by active dampers which develop a torque of opposite phase to that of the nutation torque. Several such systems use the despin motor to damp nutation. For example, in U.S. Pat. No. 4,096,427, the outputs of an accelerometer-based nutation sensor and a relative rotation rate sensor are processed to provide despin motor control signals so that the appropriate torques are applied by the despin motor to damp nutation. A second nutation control system, disclosed in U.S. Pat. No. 4,272,045, uses a horizon sensor to detect nutation. Other nutation control systems are disclosed in U.S. Pat. Nos. 3,695,554 and 3,830,447, and in Slafer and Marbach, Active Control of the Dynamics of a Dual-Spin Spacecraft", Journal of Spacecraft and Rockets, Vol. 12, May 1975, pages 287-293.
Another type of nutation control system provides active nutation damping by appropriately phased thruster pulses from one or more attitude control thrusters to provide a nutation damping torque. Such a system is shown in the paper entitled "An On-Board, Closed-Loop, Nutation Control System for a Spin Stabilized Spacecraft," by Lynn H. Grasshoff, published in the May 1968 edition of the Journal of Spacecraft and Rockets, Vol. 5, No. 5.
In order to sense or detect nutation in a dual-spin stabilized satellite, prior art nutation control systems usually employ either a rate sensor such as a gyroscope or accelerometer or a position sensor such as a horizon sensor or a beacon sensor. As mentioned above, some nutation control systems include a sensor for detecting the relative spin phase and rate between the rotor and platform. The relative spin phase sensor is necessary when the nutation sensor and torque applying means of the nutation control system are on opposite portions of the spinning satellite in order to provide the necessary information for coordinate translation of the nutation sensor signals to the coordinate system of the torque applying means.
It should be noted at this point that two goals which are always present in spacecraft design and production are to reduce the total spacecraft weight and to provide redundant or backup systems in case of a system failure. It is frequently the case that one of these goals must be sacrificed, at least partially, to achieve the other goal.